This invention relates generally to radio frequency circuits and more particularly to radio frequency (r.f.) hybrid couplers which combine or divide signals fed thereto among different ports.
As is known in the art, it is often desirable to combine a pair of r.f. signals originating from two devices and deliver such combined r.f. signal to a third device, or alternatively, to split an input r.f. signal from one device and deliver such split components of such r.f. signal to two output devices. One class of couplers includes couplers having radio frequency transmission lines formed on a substrate. In general, one component of such signal is directly coupled between one of a pair of ports and an output port, and a second one of such signals is electromagnetically coupled between a second one of such pair of ports and the output port. An approach used in the prior art to electromagnetically couple a component of such signals has been to couple an electromagnetic field between the edges of a pair of planar, dielectrically spaced strip conductors adjacently formed on a common substrate with end portions of each one of the strip conductors providing a port connection for the coupler. With this prior art approach, the strength of coupling is related to the total area of the edge and the separation between the edges of the strip conductors. As it is also known in the art, the coupler generally provides a characteristic impedance which is compatible with the circuit application of the coupler. Since coupling strength in part is related to total edge area, generally, the total edge area is increased to provide for an increase in coupling strength. One approach used in the prior art to increase the total edge area involves the technique of interdigitating a plurality of narrow strip conductors to thereby increase the total edge area and hence the coupling strength. When designing such couplers, particular attention is given to the characteristic impedance of the coupler since the coupler should provide a characteristic impedance which is compatible with the devices to which it is connected. As is also known in the art, the characteristic impedance of a transmission line, such as a microstrip transmission line, is related to substrate thickness, dielectric constant, and conductor width. Thus, the width, spacing and number of such interdigitated strip conductors are generally selected to provide the coupler with the desired coupling factor and predetermined characteristic impedance. That is, the width and number of such narrow strip conductors and their spacing are generally selected to be sufficiently narrow to provide a coupler with a desired coupling factor, and the width and number of such narrow strip conductors are likewise chosen to provide the coupler with the predetermined characteristic impedance. One problem associated with such a structure is that, as increased coupling strength is required, the conductor widths and spacing therebetween decrease providing a difficult circuit to fabricate with acceptable yields in order to achieve the desired coupling strength and to maintain the predetermined characteristic impedance. Also, as the conductor width decreases, conductor resistivity increases and hence conductor losses and therefor coupler insertion loss increases.